Multiple myeloma (MM) is an incurable malignancy of clonal plasma cells (PC). The development of MM is typically preceded by asymptomatic precursor conditions termed monoclonal gammopathy of undetermined significance (MGUS) and smoldering myeloma (SMM). Although both conditions are characterized by the accumulation of abnormal, but not yet cancerous, clonal PCs in the bone marrow (defined by <10% bone marrow PCs in MGUS vs >10% in SMM), many of these patients will never progress to MM and the frequency of their clonal BMPCs remains remarkably stable for years. However, all MGUS and SMM patients have a lifelong risk of progression to MM at a rate of ~1% and 10% per year, respectively. Because of the low transformation rate into malignancy and the toxicity of current treatments, no medical intervention is begun for MGUS or low risk SMM patients until after irreversible end-organ damage (e.g., lytic bone disease, and renal insufficiency) has occurred. It is clearly desirable to begin treatment before patients become symptomatic, but specific biomarkers that accurately predict which patients will progress to malignancy and also yield insight into the mechanisms responsible for transformation to MM do not currently exist. Given that specific trisomies and chromosomal translocations characteristic of MM PCs are already present in the clonal PCs in MGUS and SMM patients, these changes alone are not responsible for malignancy. Longitudinal studies over the past decade that have focused on identifying specific MM PC-intrinsic genomic changes that are characteristic of malignant transformation have been only marginally successful and although recurrent mutations have been identified, there is significant patient to patient heterogeneity and many unanswered questions concerning disease progression remain. The primary focus of this proposal is on changes in the BM microenvironment (ME) and the relationship between innate and adaptive immune cells in close proximity to abnormal BMPCs during disease progression from MGUS/SMM to malignant MM. The BMPC immune contexture (in situ immune cell nature, density, functional orientation, and spatial relationships) and its potential role in MGUS/SMM progression to MM have not been previously studied. Our specific hypothesis is that the BM ME actively suppresses disease progression in MGUS and SMM patients and that focal changes in the ME relieve the suppression and allow abnormal PCs in these modified niches to expand. These changes may be due to either deviations in the composition or function of the cells within the PC niche as a result of age, or may represent migration of the premalignant cells into a niche which is favorable to disease progression. We also hypothesize that changes to the BM ME may actually be driven by the abnormal PCs themselves and their release of microvesicles (MVs) that regionally expand the BMPC niche and facilitate systemic spread. This proposal will therefore focus on PC and MV-driven alterations of immune cells in the BM ME during MM progression, the functional consequences of these changes, and correlation with clinical outcome.